Probability Of Patch Occupancy Research Articles

Factors affecting presence and relative abundance of the Endangered volcano rabbit Romerolagus diazi, a habitat specialist

AbstractHabitat specialists are particularly vulnerable to extinction when habitat conditions are altered. Information on the habitat use of such species is thus important because it provides insight into factors that influence distribution and abundance, which is crucial for conservation. Here, we aimed to identify factors that influence the patterns of presence and abundance of the Endangered volcano rabbit Romerolagus diazi, a rare leporid with a patchy distribution. Through exhaustive sampling of its range in the Sierra Chichinautzin and Sierra Nevada volcanic fields, Mexico, and using generalized linear models, we found that the probability of patch occupancy was higher where bunchgrass cover exceeded 75%, rock cover exceeded 5%, no cattle grazing was observed and human settlements were at least 7 km away. Patches with greater relative abundance were those with similar characteristics, but located at elevations > 3,600 m, and with rock cover < 15%. Cattle grazing was identified as a major threat to local populations of the volcano rabbit, particularly in the Sierra Chichinautzin. Because of the significance of bunchgrasses for this species, the protection of the mountain grasslands is required in both volcanic fields.

Structural heterogeneity increases diversity of non‐breeding grassland birds

Grassland birds have experienced greater population declines than any other guild of birds in North America, and yet we know little about habitat use and the affects of management during their non‐breeding period on wintering grounds. The paucity of information on wintering grassland birds limits our ability to develop effective conservation strategies. We investigated habitat use by the winter bird community in grasslands with restored heterogeneity resulting from the interactive effects of fire and grazing. We used 500 m line transects distributed across patches (i.e., <13, 13–24, and >24 months post disturbance) resulting from spring burning with growing season grazing (April‐Sept) and quantified avian relative abundance, community structure, and probability of patch occupancy while accounting for imperfect detection. Grassland structure that resulted from the fire‐grazing interaction created heterogeneity among patches that influenced avian habitat use during winter. Generalist birds such as the Savannah Sparrow (Passerculus sandwichensis) and meadowlarks (Sturnellaspp.) were relatively common in all patch types while more specialized species such as the Smith's Longspur (Calcarius pictus) and Le Conte's Sparrow (Ammodramus leconteii) reached greatest abundance and probability of occupancy in the patches with the least and greatest time post disturbance, respectively. This research provides novel information on the response of wintering birds to restored ecological processes in grasslands and can improve efforts to create effective conservation strategies. Our findings add to a growing body of literature supporting the use of fire and grazing to create a shifting grassland mosaic that increases vegetation structural and compositional heterogeneity and maximizes native biodiversity within rangeland ecosystems through the conservation of natural patterns and processes.

Estimating breeding season abundance of golden‐cheeked warblers in Texas, USA

AbstractPopulation abundance estimates using predictive models are important for describing habitat use and responses to population‐level impacts, evaluating conservation status of a species, and for establishing monitoring programs. The golden‐cheeked warbler (Setophaga chrysoparia) is a neotropical migratory bird that was listed as federally endangered in 1990 because of threats related to loss and fragmentation of its woodland habitat. Since listing, abundance estimates for the species have mainly relied on localized population studies on public lands and qualitative‐based methods. Our goal was to estimate breeding population size of male warblers using a predictive model based on metrics for patches of woodland habitat throughout the species' breeding range. We first conducted occupancy surveys to determine range‐wide distribution. We then conducted standard point‐count surveys on a subset of the initial sampling locations to estimate density of males. Mean observed patch‐specific density was 0.23 males/ha (95% CI = 0.197–0.252, n = 301). We modeled the relationship between patch‐specific density of males and woodland patch characteristics (size and landscape composition) and predicted patch occupancy. The probability of patch occupancy, derived from a model that used patch size and landscape composition as predictor variables while addressing effects of spatial relatedness, best predicted patch‐specific density. We predicted patch‐specific densities as a function of occupancy probability and estimated abundance of male warblers across 63,616 woodland patches accounting for 1.678 million ha of potential warbler habitat. Using a Monte Carlo simulation, our approach yielded a range‐wide male warbler population estimate of 263,339 (95% CI: 223,927–302,620). Our results provide the first abundance estimate using habitat and count data from a sampling design focused on range‐wide inference. Managers can use the resulting model as a tool to support conservation planning and guide recovery efforts. © 2012 The Wildlife Society.

Patch occupancy, number of individuals and population density of the Marbled White in a changing agricultural landscape

Metapopulation theory predicts the occurrence of animals in habitat patches. In this paper, we tested predictions based on this theory, including effects of spatial autocorrelation, to describe factors affecting the presence, local number of individuals and density of the Marbled White butterfly Melanargia galathea in habitat patches spread across the agricultural landscape of southern Poland. This agricultural landscape has undergone significant changes in recent decades due to the country’s political transformation and is currently characterized by a large proportion of fallow (abandoned) land. We compared 48 occupied habitat patches with 60 unoccupied ones. Positive spatial autocorrelation was found in the number and density of individuals in habitat patches. The probability of patch occupancy was higher for patches that were larger, had a higher proportion of edges, were located closer to the nearest neighbouring local population and to the nearest piece of fallow, contained a smaller area of cut grass, and also had more nectar resources. The number of Marbled Whites in habitat patches was positively related to the patch area, the distance to the nearest fallow and the abundance of nectar resources, but was negatively related to the density of shrubs. The density of individuals was positively related to abundance of flowers, proportion of edge in a patch and distance to the nearest fallow, but it was negatively related to patch area, vegetation height and grass cover. These results indicate that recent land-use changes in agricultural landscapes have had both positive and negative effects on the presence and local number of individuals and density of the Marbled White. These changes affect the metapopulation of the species through changes in habitat quality and landscape connectivity in the area surrounding habitat patches.

Patch occupancy of North American mammals: is patchiness in the eye of the beholder?

AbstractAim Intraspecific variation in patch occupancy often is related to physical features of a landscape, such as the amount and distribution of habitat. However, communities occupying patchy environments typically exhibit non‐random distributions in which local assemblages of species‐poor patches are nested subsets of assemblages occupying more species‐rich patches. Nestedness of local communities implies interspecific differences in sensitivity to patchiness. Several hypotheses have been proposed to explain interspecific variation in responses to patchiness within a community, including differences in (1) colonization ability, (2) extinction proneness, (3) tolerance to disturbance, (4) sociality and (5) level of adaptation to prevailing environmental conditions. We used data on North American mammals to compare the performance of these ‘ecological’ hypotheses and the ‘physical landscape’ hypothesis. We then compared the best of these models against models that scaled landscape structure to ecologically relevant attributes of individual species.Location North America.Methods We analysed data on prevalence (i.e. proportion of patches occupied in a network of patches) and occupancy for 137 species of non‐volant mammals and twenty networks consisting of four to seventy‐five patches. Insular and terrestrial networks exhibited significantly different mean levels of prevalence and occupancy and thus were analysed separately. Indicator variables at ordinal and family levels were included in models to correct for effects caused by phylogeny. Akaike's information criterion was used in conjunction with ordinary least squares and logistic regression to compare hypotheses.Results A patch network's physical structure, indexed using patch area and isolation, received the greatest support among models predicting the prevalence of species on insular networks. Niche breadth (diet and habitat) received the greatest support for predicting prevalence of species occupying terrestrial networks. For both insular and terrestrial systems, physical features (patch area and isolation) received greater support than any of the ecological hypotheses for predicting species occupancy of individual patches. For terrestrial systems, scaling patch area by its suitability to a focal species and by individual area requirements of the species, and scaling patch isolation by species‐specific dispersal ability and niche breadth, resulted in models of patch occupancy that were superior to models relying solely on physical landscape features. For all selected models, unexplained levels of variation were high.Main conclusions Stochasticity dominated the systems we studied, indicating that random events are probably quite important in shaping local communities. With respect to deterministic factors, our results suggest that forces affecting species prevalence and occupancy may differ between insular and terrestrial systems. Physical features of insular systems appeared to swamp ecological differences among species in determining prevalence and occupancy, whereas species with broad niches were disproportionately represented in terrestrial networks. We hypothesize that differential extinction over long time periods in highly variable networks has driven nestedness of mammalian communities on islands, whereas differential colonization over shorter time‐scales in more homogeneous networks probably governed the local structure of terrestrial communities. Our results also demonstrate that integration of a species' ecological traits with physical features of a patch network is superior to reliance on either factor separately when attempting to predict the species' probability of patch occupancy in terrestrial systems.

A landscape‐scale test of the predictive ability of a spatially explicit model for population viability analysis

Summary Although population viability analysis (PVA) is widely employed, forecasts from PVA models are rarely tested. This study in a fragmented forest in southern Australia contrasted field data on patch occupancy and abundance for the arboreal marsupial greater glider Petauroides volans with predictions from a generic spatially explicit PVA model. This work represents one of the first landscape‐scale tests of its type. Initially we contrasted field data from a set of eucalypt forest patches totalling 437 ha with a naive null model in which forecasts of patch occupancy were made, assuming no fragmentation effects and based simply on remnant area and measured densities derived from nearby unfragmented forest. The naive null model predicted an average total of approximately 170 greater gliders, considerably greater than the true count (n = 81). Congruence was examined between field data and predictions from PVA under several metapopulation modelling scenarios. The metapopulation models performed better than the naive null model. Logistic regression showed highly significant positive relationships between predicted and actual patch occupancy for the four scenarios (P = 0·001–0·006). When the model‐derived probability of patch occupancy was high (0·50–0·75, 0·75–1·00), there was greater congruence between actual patch occupancy and the predicted probability of occupancy. For many patches, probability distribution functions indicated that model predictions for animal abundance in a given patch were not outside those expected by chance. However, for some patches the model either substantially over‐predicted or under‐predicted actual abundance. Some important processes, such as inter‐patch dispersal, that influence the distribution and abundance of the greater glider may not have been adequately modelled. Additional landscape‐scale tests of PVA models, on a wider range of species, are required to assess further predictions made using these tools. This will help determine those taxa for which predictions are and are not accurate and give insights for improving models for applied conservation management.